Determine how many channels your amplifier has. Typically, for one subwoofer, you would prefer a 2-channel or mono-channel amplifier. This should be easily identifiable, either by the printing on the amplifier or in the amplifier's manual, or by simply counting the number of positive and negative terminals on the amplifier. If connecting a subwoofer to a 4-channel amplifier, you will almost always want to bridge the rear terminals on the amplifier by creating one output from two sets of terminals, doubling the power.
Decide whether or not to bridge the connection. Bridging a connection will double the power, so in most circumstances you will want to bridge. To bridge a connection, you will simply wire the positive speaker wire to the positive terminal on one of the two sets of terminals on the amplifier. You will then connect the negative speaker wire to the negative terminal on the other set of terminals on the amplifier.
Strip the plastic casing from the other ends of the speaker wires and connect them to the terminals on the subwoofer enclosure. Be sure to connect the wires to the corresponding charge--red means positive, black means negative.
Adjust the low pass filter on your amplifier--a switch and typically a dial--in order to improve sound quality from your subwoofer. Do this according to the specifications provided in your subwoofer manual, or you can listen and tweak until you arrive at your desired sound.